Most dimmable LED drivers have a linear dimming curve. Typically the dimming level is coded over 8 bits. The human eye response is however essentially non-linear. Therefore, steps in the lowest values are perceived much more strongly than steps in highest values. The fast response of LEDs does not intrinsically smoothen the perception of the steps.
To prevent visible steps, some drivers add a fade between the steps. This is needed for the lower level steps, but not for the higher levels. Most of the time a fade time value is selected that is a compromise between a change in brightness that is fast enough and steps which cannot be perceived. For example, a fade period may have a duration of the order of 30 ms to 100 ms.
The fade function between dimming levels aims to mimic tungsten sources.
The fade function operates with a fixed time constant, resulting in a slower behavior of the light output under certain dimming change conditions. For example, the dimming level may be set every 20 ms for a 50 Hz system. If a total fade period of 100 ms is provided, the difference between the new dimming level and the old dimming level can be divided by 5, to provide 5 equal step changes in dimming level over the total 100 ms time period allocated to a change in brightness.
US 2011/0068689 discloses a dimming control system, in which it is recognized that more gradual changes in brightness are desired at lower brightness levels than at higher brightness levels. Thus, within a fixed time period over which the dimming level is changed, the brightness follows an exponential curve between a low and a high brightness. The exponential curve is approximated by segments corresponding to a sampling rate (e.g. 25 Hz). The fixed time period may have one value for a brightness increase and a different value for a brightness decrease.
The use of a fixed time period means the response may be too slow in some situations and not slow enough in others.